Method of producing a metallic multilaminated tube



Oct. 6, 1970 E s ETAL 3,532,476

METHOD OF PRODUCING A METALLIC MU'IILAMINATED TUBE Filed Sept. 30, 1969United States Patent US. Cl. 2925.13 6 Claims ABSTRACT OF THE DISCLOSUREMethod according to Pat. No. 3,461,523 includes applying both to theelongated metallic member, before spraying the latter with the layer ofinsulating material, and to the layer of insulating material thereafter,at least one layer, respectively, of metal having a lower melting pointthan that of the insulating layer and relatively good wettingcharacteristics with respect to the metal material in engagementtherewith.

Our invention relates to method of producing a metallic multilaminatedor compound tube and more particularly to a method according to our Pat.No. 3,461,523 issued Aug. 19, 1969.

In our just-mentioned patent, we disclose a method of producing ametallic multilaminated tube having at least one electrically insulatinglamination wherein we spray a layer of insulating material on anelongated metallic member and move the insulation-coated metallic memberand a hollow metallic tube, having an inner diameter greater than theouter diameter of the insulation-coated member, axially relative to oneanother so as to locate the metallic tube around the insulation-coatedmember. We then heat the metallic tube in zones progressively along thelength thereof to a temperature at which the material of the tube is inplastic state and simultaneously apply tensile stress to the metallictube in the longitudinal direction thereof so as to deform the plasticmaterial in the heated zones whereby the metallic tube is shrink-fittedonto the insulation-coated metallic member. The insulating layerconsists, for example, of A1 0 ZrO or BeO and is applied to the metallicmember by a conventional plasma spraying technique. This method of ouraforementioned patent is especially suited for producing anode tubes ofcylindrical thermionic converters which are formed, for example, of arespective inner and outer niobium tube as well as of a layer ofinsulation, suitably ground or cut in accordance with its purpose,disposed between the two niobium tubes.

Such multilaminated or compound tubes must possess as high a thermalconductivity as possible in addition to having adequate insulatingresistance and, as a further requirement, these characteristics shouldremain constant over very long periods of time i.e. even for years.

It is an object of our invention to improve the heat passage orconduction through such multilaminated or compound tubes.

With the foregoing and other objects in view, after furtherexperimentation, we have succeeded in markedly improving the thermalconductivity of such tubes by providing a method according to ouraforementioned patent including the additional steps of applying both tothe elongated metallic member, before spraying the latter with the layerof insulating material, and to the layer of insulating materialthereafter, at least one layer, respectively, of metal having a lowermelting point than that ice of the insulating layer and relatively goodwetting char acteristics with respect to the metal material inengagement therewith.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as method ofproducing a metallic multilaminated tube, it is nevertheless notintended to be limited to the details shown, since various modificationsmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalents of the claims.

The invention, however, together with additional objects and advantagesthereof will be best understood from the following description when readin connection with the accompanying drawing, in which:

FIG. 1 is a sectional view of a multilaminated tube produced inaccordance with the method of our Pat. No. 3,461,523 and a component ofthe apparatus employed in carrying out that method;

FIG. 2 is a view similar to FIG. 1 showing the multilaminated tubeproduced by the improved method of th instant application; and

FIG. 3 is a fragmentary enlarged sectional view of the encircled regionshown in FIG. 2.

Referring now to the drawing and first, particularly, to FIG. 1 thereof,there is shown a multilaminated threelayer tube produced by the methodof our aforementioned patent. The tube of FIG. 1 is made up of an innerhollow tube 1 of niobium, for example, such as the anode of a thermionictube, which is coated with a layer 3 of aluminum oxide by a conventionalplasma spray method, for example. After the desired thickness of theinsulating layer has been achieved, it is externally worked by polishingso that a close union is afforded to a tube 2, also for example ofniobium, that is thereafter fitted thereon. The tube 2 initially has aslightly larger diameter than the previously coated inner anode tube butis heated at progressive locations alon the length thereof by ahighfrequency induction coil 4 to a temperature at which the tube 2attains a substantially plastic state, andtensile stress is applied inthe direction of the arrow P simultaneously for deforming the respectiveplastic portion of the tube 2 and tightly fitting the tube onto thepreviously formed two-layer tube 1, 3. Continued relative displacementof the tube assembly 1, 3, 2 and the heating coil 4 in the axialdirections thereof causes the tube 2 to e completely shrunk onto theinsulation-coated inner tube 1. This assembly, which is especiallysuitable for thermionic converters employed Within nuclear reactors,must be operable uninterruptedly for at least two years during which itis cooled by liquid sodium to a temperature of about 700 C. The innersurface of the multilaminated tube assembly forms one wall of theconverter gap wherein cesium having a vapor pressure of about 4 torr iscontained. The temperature differential between the inner and outer wallthereof should not exceed 1 C. for a heat flow of 1 watt per squarecentimeter.

With the multilaminated tube produced according to the method of ouraforementioned patent, the temperature differential across the inner andouter walls was found to be from 3 to 4.5 C., however. Accurate testsdisclosed that this rather poor thermal conductivity was due toinsufficient thermal contact between the metal and the A1 0 layerresulting from the following causes. When a layer 3 of A1 0 was appliedto the hollow metal tube 1 it was necessary to completely melt the A1 0particles, requiring for example the expenditure of 12 kw. heat energy.This was possible, however, only if nitrogen were employed as workinggas. This gas formed a nitride layer on the niobium tube 1 which becamedecomposed when cooled, causing a decrease in thermal contact between 3the niobium tube 1 and the A1 layer 3 due to the formation of a fine,i.e. narrow, gap or space between the adjacent surfaces thereof. Thisphenomenon was found to be preventable according to our invention byapplying a layer 52, according to one example of our invention, of analloy Nlgo'crgo, having a melting point of 1440 C., to the niobium tube1 before spraying the insulating layer 3 thereon. In accordancewithanother feature of the invention and in order to achieve adequateWetting of the niobium tube 1 with this alloy, a thin coating of copperis first applied to the niobium tube 1 by a spray technique orgalvanically. When the insulating layer 3 was subsequently applied byplasma spraying, the copper coating 51 melted momentarily and wetted thesurface of the niobium tube 1 as well as the opposing surface of theNiCr layer 52. The Ni-Cr layer 52 was not attacked by the nitrogenworking gas and was partly melted by the application of the ceramic A1 0layer so as to provide thereby a relatively good contact with the A1 0particles.

In a similar manner, the thermal connection or bond between theinsulating layer 3 and the outer niobium tube 2 is produced. Thenickel-chrome alloy is again initially sprayed onto the insulating layer3 to form a layer 61 thereon, and a thin copper layer 62 is againapplied to the nickel-chrome layer 61. Thereafter, the outer niobiumtube 2 is sweated onto the thus-formed mutilaminated tubular assembly 1,51, 52, 3, 61, 62 in the manner disclosed in our Pat. No. 3,461,523. Theouter copper layer 62 accordingly melts and, upon cooling, in effectsolders the nickel-chrome layer 61 to the outer niobium tube 2.

It is possible to produce multilaminated or compound tubes by theimproved method invention of the instant application practically of anydesired length wherein, for example, the spacing between the tubes 1 and2 can be extremely small, such as for example of the order of magnitudeof from 0.2 to 0.3 mm. The thickness of the A1 0 insulating layer isthen about 0.15 to 0.2 mm.

Collector tubes produced by the method of the instant applicationexhibit a temperature differential of from 0.7 to 1 C. with respect tounity of the heat flow density and consequently meet the aforementionedrequirement.

The aforedescribed example taken from the technology of thermionicconverters indicates quite clearly the great demands with respect toinsulation stability and thermal conductivity that can be satisfied bythe method of the invention in this application. It is of course quiteobvious that the method of this invention is applicable as well to othertechnological arts.

We claim:

1. In a method of producing a multilaminated tube having at least oneelectrically insulating lamination by spraying a layer of insulatingmaterial on an elongated metallic member, moving the insulation-coatedmetallic member and a hollow metallic tube, having an inner diametergreater than the outer diameter of the insulationcoated member, axiallyrelative to one another so as to locate the metallic tube around theinsulation-coated member, heating the metallic tube in zonesprogressively along the length thereof to a temperature at which thematerial of the tube is in plastic state, and simultaneously applyingtensile stress to the metallic tube in the longitudinal directionthereof, so as to deform the plastic material in the heated Zoneswhereby the metallic tube is shrink-fitted onto the insulation-coatedmetallic member, the improvement which comprises applying both to theelongated metallic member, before spraying the latter with the layer ofinsulating material, and to the layer of insulating material thereafter,at least one layer, respec tively, of metal having a lower melting pointthan that of the insulating layer and relatively good wettingcharacteristics with respect to the metal material in engagement 1 metallayer applied to the metallic member being resistant to attack by theworking gas.

3. Method according to claim 2 which comprises initially applying to theelongated metallic member a coating of metal having good wettingcharacteristics relative to the material of the elongated metallicmember.

4. Method according to claim 3 wherein the metal layer applied to theinsulating layer is resistant to attack by the working gas.

5. Method according to claim 4, which comprises ap plying to the metallayer initially applied to the insulating layer a coating of metalhaving good wetting characteristics relative to the material of themetallic tube.

6. Method according to claim 5 wherein the working gas is nitrogen, theelongated metallic member and the metallic tube are formed of niobium,the metal coatings having good wetting characteristics are formed ofcopper, the metal layers resistant to attack by the nitrogen are formedof a niobium-chromium alloy, and the insulating material is a ceramicselected from the group consisting of A1 0 ZrO and BeO.

References Cited UNITED STATES PATENTS 2,609,595 9/ 1952 Rossheim.

2,781,308 2/1957 Creutz et al.

3,009,484 11/1961 Dollens.

3,010,355 11/1961 Cutforth.

3,279,028 10/1966 Hall et al 310-4 X 3,318,340 5/1967 Torti 138-1403,368,084 2/1968 Hall 310-4 3,461,523 8/1969 Peehs et a1 29-25.l3

CHARLIE T. MOON, Primary Examiner U.S. Cl. X.R.

Patent No. 3,532,476 Dated October 1-970 Inventor(s) Martin Peehs andHeinz SchBrner It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the heading, page 1, column 1 the German priority number should read--P 18 00 307.1--

SIGNED mu QEALEF Atteat:

Edward M. Fletcher, Ir. WIMIAM E. SGHUYLER, JR. L Ane ti Officommissioner of Patents

